Some carburetors include a priming pump with a flexible bulb that can be depressed to force fluid from the bulb into the carburetor to prime the carburetor. In one carburetor, fluid discharged from a priming pump flows through a passage that directs the fluid away from a fluid supply pipe of the carburetor through which fuel is supplied to a fuel and air mixing passage for subsequent delivery to an engine in a fuel and air mixture. The flow of fluid away from the fuel supply pipe does not effectively remove air from the fuel supply pipe or direct fuel into and through the fuel supply pipe such as to provide a richer than normal fuel and air mixture to the engine to facilitate starting it.
In another carburetor construction, when the priming pump bulb is actuated or depressed, air is discharged under pressure from the priming pump to an atmospheric air space above the fuel level of a float chamber. Prior to depressing the bulb, an atmospheric vent is closed so that a super atmospheric pressure is developed in the air space when the air is discharged therein by the priming pump. The super atmospheric pressure in the air space acts on the fuel in the fuel chamber and causes fuel in the float chamber to flow into the fuel supply pipe to prime the carburetor. A large-capacity priming pump is required to sufficiently pressurize the atmospheric air space above the fuel level of the float chamber. Further, the operation of the priming pump in this carburetor is not easily controlled resulting in overly lean or overly rich fuel and air mixtures delivered to an engine upon starting the engine. The undesirable fuel and air mixtures result in failure to start the engine, or in a start and idle of the engine that is not stable or cannot be maintained.
In one construction of a carburetor of this type, the atmospheric vent for the air space of the float chamber is provided in the priming pump bulb. While the vent may be conveniently closed by a finger of an operator when the priming pump is actuated, the open vent permits contaminants such as dust, dirt and water to enter the vent when it is not closed.